1. Technical Field
The present invention relates to a solid-state imaging device suitable for use in an apparatus using both rolling and frame shutter modes.
2. Related Art
A CCD (charge-coupled device) type image sensor (hereinafter referred to as a “CCD sensor”) and a CMOS type image sensor (hereinafter referred to as a “CMOS sensor”) are used as solid-state imaging devices incorporated in cellular phones, digital cameras and the like. In addition, a MOS type solid-state imaging device (hereinafter referred to as a “substrate modulation sensor”) has been proposed that uses a threshold-voltage modulation system combining high image quality with low power consumption. CMOS sensors and substrate modulation sensors (hereinafter collectively referred to as “CMOS sensors and the like”) are recently being developed because of their benefits of lower power consumption and process cost compared to CCD sensors.
Some of the CMOS sensors and the like employ a rolling shutter mode in which image signals are read out line by line, while others of them employ a frame shutter mode in which light-generated charges are stored simultaneously in many light receiving elements that are arranged two-dimensionally.
Meanwhile, in the CMOS sensors and the like, in order to remove noise components, correlated double sampling (CDS) is performed. In this method, both a pixel output after light reception by a photodiode and a pixel output after pixel-charge clearance (reset) are sampled to obtain a difference between the outputs.
By way of example using a type of CMOS-APS (Active Pixel Sensor) having four transistors, a brief explanation will be provided as below, regarding the double sampling process of a CMOS sensor in the rolling shutter mode. First, in order to read out a noise component, a floating diffusion, which is a charge accumulation region, is reset. Next, a potential based on a light-generated charge remaining in the floating diffusion region is output (readout of a noise component). After this, a light-generated charge generated by a photodiode is transferred to the floating diffusion region. Then, a potential based on the light-generated charge transferred thereto is output (readout of a signal component). The noise component is removed by processing a difference between the readout noise and signal components.
In the CMOS sensors and the like as shown above, an image signal-readout operation is performed line by line, and so is noise readout. That is, pixel reset (clearance) is also done line by line. In the rolling shutter mode, transfer of a light-generated charge generated by a photodiode to a floating diffusion is also performed line by line. Accordingly, as described above, noise components can be read out before the readout of signal components.
On the other hand, in the frame shutter mode, light-generated charges of all pixels are transferred collectively to the floating diffusion region. Accordingly, noise components cannot be read out before the readout of signal components. In other words, it is necessary for CMOS sensors and the like employing the frame shutter mode to repeat a line-by-line operation including a signal readout of a line, a pixel reset of a readout line and a noise readout of the line after transferring the light-generated charges of all the pixels to the floating diffusion region.
As mentioned above, in the CMOS sensors and the like, depending on which of the shutter modes is employed, the readout order between a noise component and a signal is reversed.
Japanese Patent No. 2965777 is an example of related art.
The difference between a noise component and a signal component can be detected by a circuit disposed at each readout signal line. In the example of related art, a noise component is removed by an FPN (fixed pattern noise) suppression circuit disposed at a vertical signal line VL.
An FPN suppression circuit, which is composed of a plurality of capacitive elements and an inversion amplifier, detects a difference between the noise component and a signal component to output a signal after a noise removal. The FPN suppression circuit uses an inversion amplifier. Thus, based on a threshold of the inversion amplifier, the circuit outputs a signal whose output level increases in a direction opposite to a changing direction of an input signal.
For example, if the output level of a component Vo based on a noise readout is higher than that of a component Vps based on a signal readout, in the rolling shutter mode giving precedence to a noise readout, the level of a signal input to the FPN suppression circuit drops from a high to lower value. Consequently, the level of a signal output from the FPN suppression circuit increases in accordance with the reduced level of the input signal
On the contrary, the frame shutter mode prioritizes readout of a signal component. Thus, the level of a signal input to the FPN suppression circuit increases from a low to higher value. As a result, the level of a signal output from the FPN suppression circuit drops to a lower value in accordance with the increased level of the input signal.
That is, based on the threshold of the inversion amplifier, in the rolling shutter mode, the output voltage of the FPN suppression circuit increases in the positive side, whereas in the frame shutter mode, the output voltage thereof increases in the negative side.
Therefore, in the case of the CMOS sensors and the like using both rolling and frame shutter modes, the output level of an FPN suppression circuit is likely to deviate from a dynamic range of the circuit.
In the FPN suppression circuit used in the above example of related art, the dynamic range thereof can be set so as to be corresponding to each shutter mode by switching a threshold voltage applied to the inversion amplifier. In this case, however, a plurality of threshold voltages needs to be prepared to switch, resulting in an increase in the circuit size.